A replicated study in 1993-1999 in boreal forest in British Colombia, Canada (1) found that plastic mulch mats decreased the total cover of herbaceous species and grasses in the first three years after treatment, but cover was similar to control plots 5-7 years after treatment. The total cover of grasses and herbaceous species was lower in plots with mulch mats (39-33%) than in control plots (71-68%) in the first three years, but similar 5-7 years after treatment (mulch: 70-90%; control: 80%). Herbaceous species and grasses were monitored in four 12 × 12 m plots of each treatment: control and covered with 90 × 90 cm plastic mulch mats. The study site had been planted with Douglas-fir Pseudotsuga menziesii in 1993.